I have looked at the RCP Text Editor example. I have an existing plug-in with an editor that I wish to optionally make available as an RCP editor (i.e. you can use the plugin or you can just use the editor separately).

I was wondering if I can simply add the Application bits to the existing plug-in and then build it as an RCP application or do I need a separate plug-in which references the classes of the original plugin and duplicates a number of its extensions?

you can turn that single plugin into an RCP app, with a little extra
work. Generate the RCP Mail app and then have a look at that (it's a
single plugin that provides a perspective, some views, and an RCP app).